Heating the aluminium ore with concentrated caustic soda.

Explanation

Aluminium oxidide being amphoteric in nature, it react with the concentred solution of sodium hydroxide (caustic soda) at `140^(@)C "to" 150^(@)C` under high pressure for 2 to 8 hours in a digester to form water soluble sodium aluminate.
`underset("Aluminium oxide")(Al_(2)O_(3).2H_(2)O_((s)))+underset("Sodium hydroxide")(2NaOH_((aq)))to underset("Sodium aluminate")(2NaAlO_(2(aq)))+underset("Water")(3H_(2)O_((l)))`